In the prior art there are known methods of producing magnesium sulphate contemplating the processing of natural polymineral ores or brines comprised of magnesium sulphate, with ferrous sulphate being employed as a sulphate-containing component, and the process of thermal treatment being conducted at a temperature ranging from 400.degree. to 900.degree. C.
The lengthy processes and the necessity of use of the corrosion-resistant equipment required for practicing these prior art methods should be considered as apparent disadvantages. Moreover, while practicing the foregoing methods ferrous sulphate is to be employed in crystalline form, which requires an additional expenditure of energy for its recovery from the effluent solutions.
Also known in the prior art is a method of processing pickling lyes with a suspension of magnesium hydroxide and calcium hydroxide, residing in that by means of interacting pickling solutions comprises of free sulphuric acid and ferrous sulphate with active magnesium and calcium hydroxides, the resulting solution of magnesium and calcium sulphates is formed, which solution is treated by burnt and hydrated dolomitic lime with the formation of gypsum and active magnesium hydroxide, a portion of which is returned to the cycle.
The disadvantages inherent in the above described method consist in its multiplicity of stages and complexity encountered in carrying out the procedures, as well as its low efficiency due to the use of active magnesium hydroxide and the effectuation of the process in the highly diluted solutions.
Also known in the prior art is a method of producing magnesium sulphate by means of exchange interaction of magnesium compounds with ferrous sulphate (cf. e.g., Inventor's Certificate No. 384793).
A homogenized dry mixture of ferrous sulphate and magnesium carbonate is calcinated at a temperature of 500.degree. to 900.degree. C.
Subsequently, magnesium sulphate is lixiviated by water, the suspension is filtered and the ultimate product is separated from the solution.
At the present time it has been found that at a temperature of 80.degree.-160.degree. C. in treating a mixture of a heptahydrate of ferrous sulphate and magnesium carbonate, the heptahydrate of ferrous sulphate is dehydrated first to a tetra, and then to a monohydrate. Further heating to 400.degree. C. causes the monohydrate to be dehydrated and the iron to be oxidized, with ferrous sulphate and hydrosulphate being produced. The process of interaction of the ferrous sulphates with magnesium carbonate is intensive enough only over the range of temperatures from 550.degree. to 700.degree. C. The highest degree of conversion (about 70%) of magnesium carbonate on interacting with ferrous sulphate to magnesium sulphate takes place at a temperature of 650.degree. C.
The degree of conversion is defined as a ratio of the amount of magnesium transformed to magnesium sulphate to the amount of magnesium comprised in the starting material.
The disadvantages intrinsic in the foregoing method lie in the complexity of its implementation due to the impossibility of recovering magnesium sulphate as an ultimate product, inasmuch as the use of active magnesium hydroxide in the process is indispensable to the accomplishment of the method, which substance is highly difficult to obtain in practice, and therefore magnesium sulphate produced by this method is merely an intermediate product.
On thermal interaction of ferrous sulphate with magnesium carbonate, ferrous sulphate is intensively decomposed at a temperature over 500.degree. C. with the formation of sulphur oxides, which causes the combustible gaseous mixtures to become contaminated with the latter. For example, at 600.degree. C. up to 30% of ferrous sulphate is decomposed with the release of sulphur oxides, while the process of thermal treatment is under way. In case of the use of commercial copperas instead of pure ferrous sulphate, for instance, a waste product of the manufacture of titanium dioxide, the process of decomposing the sulphates is even more intensive and occurs at a lower temperature.
Consequently, the above-described prior art method is industrially impracticable.
Another disadvantage of this prior art method, which also makes its industrial application unfeasible, lies in the fact that the method can be realized only on condition that small amounts of pure ferrous sulphate with magnesium carbonate are properly grounded and thermally treated.